Recently, demand for plated steel sheets, improved in terms of corrosion resistance and aesthetic appearance has increased, for example, for use in applications such as electrical appliances and automobiles.
For example, FIG. 1 illustrates hot dipping equipment such as hot-dip galvanizing equipment for galvanizing steel sheets.
Referring to FIG. 1, a steel sheet S (e.g., a hot-rolled steel sheet S) unwound from a pay-off reel is carried through a welder and a looper and is heat-treated. Then, while the steel sheet S passes through a snout and a zinc plating bath 110, molten zinc ZL is applied to the steel sheet S. At this time, gas wiping devices (air knives) 100 disposed in the zinc plating bath 110 blows gas (such as inert gas or air) to the surfaces of the steel sheet S so as to control (adjust) the thickness of a plating layer (that is, a zinc plating layer) formed on the steel sheet S by partially removing the plating layer.
Thereafter, the steel sheet S is carried along a cooling device, carrying rolls, and a plating measurement unit 130. The amount of plating measured when the steel sheet S passes through the plating measurement unit 130 is feedback to adjust a gas blowing pressure of the gas wiping devices 100 or the distances (gaps) between the gas wiping devices 100 and the steel sheet S to thus control the amount of plating (that is, the thickness of the plating layer) by a feedback method.
In FIG. 1, reference numerals 112 and 114 refer to a sink roll and a stabilizing roll for stretching a steel sheet S and adjusting tension of the steel sheet S.
The gas wiping devices 100 are main devices of plating equipment by which the thickness of a plating layer is mostly affected, and the thickness of a plating layer is a main factor determining the quality of plating.
Referring to FIG. 2, in each the gas wiping devices 100 illustrated in FIG. 1, a device nozzle 101 including upper and lower lips 103 and 104 forming a gas outlet 102 is attached to a device main body (chamber) 105 in the form of flanges F, and a high-pressure gas supply pipe 106 is connected to the device main body 105. In addition, a rectifying plate 107 and a mesh 108 may be disposed between the device main body 105 and the device nozzle 101.
As shown in FIG. 2, if a high-pressure gas (wiping jet J) discharged through the gas outlet 102 of the device nozzle 101 collides with a surface of a plated steel sheet S, the wiping jet J may be divided into upward and downward wall-surface jets J along the surface of the plated steel sheet S. Then, while the wall-surface jets J move rapidly along a hot-dip zinc plating layer ZL formed on the steel sheet S, the hot-dip zinc plating layer may be partially removed, and thus the amount of plating on the steel sheet S may be adjusted.
Recent steel sheet plating processes are required to form a thin plating layer on a steel sheet S while moving the steel sheet S at high speed so as to increase productivity. That is, if a steel sheet S is coated with a thin plating layer by performing a plating process only to a necessary degree, the manufacturing costs of the plated steel sheet S may be reduced, and the productivity of the plating process may be improved.
However, if a steel sheet is moved at high speed, each of the gas wiping devices 100 is required to discharge a wiping jet J having large momentum so as to thin a plating layer. That is, the gas pressure or flow rate of the gas wiping device 100 may need to be increased to increase the gas wiping capacity thereof.
As shown in FIG. 2, generally, the amount of plating on a steel sheet S is adjusted by varying the pressure of gas at the device nozzle 101 or the distance between the device nozzle 101 and the steel sheet S.
High productivity, for example, rapid formation of a thin plating layer, may be obtained by increasing the pressure or flow rate of wiping gas.
However, if the ability of wiping is improved by increasing the pressure or flow rate of gas for the rapid formation of a thin playing layer, the scattering of zinc particles P, known as splashing, may be increased as compared with the case of low-speed plating, and thus a large amount of top dross D may be formed above the surface of molten zinc of the plating bath 110.
That is, if the line speed of a steel sheet is increased and the pressure or flow rate of gas is accordingly increased for forming a thin plating layer with high productivity and low costs, scattering of particles is adversely increased. Therefore, there is a practical limit to increasing the line speed of a steel sheet.
For example, if the line speed of a steel sheet is 140 mpm in a plating process, dross may be generated at a rate of about 0.4 ton/hr because of scattered particles. However, if the line speed of a steel sheet is increased to 180 mpm in a plating process to increase the productivity of the plating process, the generation rate of dross may be markedly increased to about 1.4 ton/hr. That is, since a high line speed of a steel sheet requires a high pressure in wiping gas and markedly increases scattering of particles and the formation of dross, there is a limit to increasing the line speed of a steel sheet in a steel sheet plating process.
If the scattering of particles (i.e., scattering of zinc particles) increases, it may be difficult to perform a process at high speed in a continuous galvanizing line (CGL), and thus the productivity of the CGL may be lowered. Particularly, a steep increase in the amount of top dross D may cause contamination of devices such as rolls disposed in a plating bath or may worsen the plating quality of a steel sheet. Thus, an additional process may be necessary to remove such dross. However, this may increase the workload of workers.